Processability is the ability to economically process and shape a polymer uniformly. Processability involves such elements as how easily the polymer flows, melt strength, and whether or not the extrudate is distortion free. Typical bulky ligand metallocene-type catalyzed polyethylenes (mPE) are somewhat more difficult to process than low density polyethylenes (LDPE) made in a high pressure polymerization process. Generally, mPE's require more motor power and produce higher extruder pressures to match the extrusion rate of LDPE's. This is typically evident where a polymer exhibits a low melt index ratio. Typical mPE's also have lower melt strength which, for example, adversely affects bubble stability during blown film extrusion, and they are prone to melt fracture at commercial shear rates. On the other hand, however, mPE's exhibit superior physical properties as compared to LDPE's.
It is now common practice in the industry to add various levels of an LDPE to an mPE to increase melt strength, to increase shear sensitivity, i.e., to increase flow at commercial shear rates; and to reduce the tendency to melt fracture. However, these blends generally have poor mechanical properties as compared with neat mPE.
Traditionally, metallocene catalysts produce polymers having a narrow molecular weight distribution. Narrow molecular weight distribution polymers tend to be more difficult to process. The broader the polymer molecular weight distribution the easier the polymer is to process. A technique to improve the processability of mPE's is to broaden the products' molecular weight distribution (MWD) by blending two or more mPE's with significantly different molecular weights, or by changing to a polymerization catalyst or mixture of catalysts that produce broad MWD polymers.
In the art specific bulky ligand metallocene-type catalyst compound characteristics have been shown to produce polymers that are easier to process. For example, U.S. Pat. No. 5,281,679 discusses bulky ligand metallocene-type catalyst compounds where the bulky ligand is substituted with a substituent having a secondary or tertiary carbon atom for the producing of broader molecular weight distribution polymers. U.S. Pat. No. 5,470,811 describes the use of a mixture of bulky ligand metallocene-type catalysts for producing easy processing polymers. Also, U.S. Pat. No. 5,798,427 addresses the production of polymers having enhanced processability using a bulky ligand metallocene-type catalyst compound where the bulky ligands are specifically substituted indenyl ligands.
A need exists in the industry for catalyst compounds and catalyst systems that in a polymerization process produce polymers having a combination of properties for use in various end-use applications.